1. Field of the Invention
The present invention relates to a three-piece oil-ring consisting of a pair of side rails and an expander provided between the paired side-rails.
2. Description of the Prior Arts
Conventional three-piece oil-rings include an expander extending in a circumferential direction and having waves in a radial direction. Both ends of the expander are opposed to each other thereby producing a spring characteristic. It has been necessary to provide a hole in the expander between a radially inner portion and a radially outer portion of the waving form of the expander in order to pass a wire for preventing the ends of the expander from overlapping.
In order to suppress abrasion between a side rail engaging portion formed at a radially inner portion of the expander and a side rail, the circumferential length of the side rail engaging portion has been increased thereby increasing the contact surface area between the side rail engaging portion and the side rail. However, when the circumferential length of the side rail engaging portion of the expander is increased, the radially outer portion of the waving expander must be bent with a relatively tight curvature, i.e., having a small radius. Therefore, when a hole for passing a wire is provided in the expander which has bent sharply, the hole weakens the strength of the expander and durability of the expander is reduced.
On the other hand, if a hole for passing a wire is not provided in the expander, abutted ends of the expander may overlap in the circumferential direction when the expander is in an oil-ring groove of a piston. Such overlapping of the ends of the expander will deteriorate smooth insertion of a piston into a cylinder bore or deteriorate the expanding spring characteristic of the expander thereby increasing oil consumption.